Great game! Very clear interpretation of the theme - Killing enemies costs you space in the arena when they die and explode, using missiles (or being shot at by missiles) reduces the arena, and choosing to either save currency for progressing, or spend it it get permanent upgrades.
Graphical style was great, really evoked the PS1 era, and a great variety of enemy models, each with a different combat style was great keeping you guessing every combat.